Organic electroluminescent devices using an organic substance are promising in applications as a cheap large-area full-color display device having a solid light-emitting solid-state device and a light source array for writing, and a number of developments have been carried out. In general, an organic electroluminescent device is constructed of a light emitting layer and a pair of counter electrodes disposed sandwiching the light emitting layer therebetween. When an electric field is applied between the both electrodes, an electron is injected from the cathode, and a hole is injected from the anode. The light emission is a phenomenon in which the electron and the hole are re-coupled in the light emitting layer, and energy is released as light during a time when the energy level is returned to a valence band from a conductor.
However, in the case of such an organic electroluminescent device, there is a serious problem that the luminous efficiency is very low as compared with inorganic LED devices and fluorescent tubes.
Almost all of organic electroluminescent devices which are currently proposed are ones utilizing fluorescent light emission obtained by a singlet exciton of an organic light emitting material. In a simple mechanism of the quantum chemistry, in the exciton state, a ratio of the singlet exciton from which fluorescent light emission is obtainable to the triplet exciton from which phosphorescent light emission is obtainable is 1/3. So far as the fluorescent light emission is utilized, only 25% of the exciton can be effectively applied so that the luminous efficiency is low.
On the other hand, if phosphorescence obtainable from the triplet exciton can be utilized, the luminous efficiency should be able to be enhanced. Actually, in recent years, organic electroluminescent devices utilizing phosphorescence with a phenylpyridine complex of iridium have been reported, and it is reported that such organic electroluminescent devices exhibit the luminous efficiency of 2 to 3 times as compared with the conventional organic electroluminescent devices utilizing fluorescence (for example, see U.S. Pat. No. 6,303,238, Applied Physics Letter, 1999, Vol. 75, page 4, and Japanese Journal of Applied Physics, 1999, Vol. 38, pages L1502 to L1504).
Most of phosphorescence organic electroluminescent devices have a device construction of anode/hole transport layer/light emitting layer/block layer/electron transport layer/cathode. Here, the hole transport layer is a layer for transporting a hole from the anode into the light emitting layer. The electron transport layer is a layer for transporting an electrode from the cathode into the light emitting layer. One of works of the block layer is to block diffusion of a triplet exciton formed in the light emitting layer, and the other work is to block a phenomenon that the hole passes through the light emitting layer into the electron transport layer and the cathode. In this way, by blocking the triplet exciton or the hole or the both by the block layer, it is possible to design to enhance the luminous efficiency.
As a material to be used in the block layer, electron transport materials having a large energy level T1 that is a lowest energy level of the triplet excited state are chosen for the purpose of blocking the triplet exciton, and electron transport materials having a large ionization potential are chosen for the purpose of blocking the hole.
Bathocuproine (BCP) is a representative material. The T1 of BCP is large as 2.5 eV, and its ionization potential is large as 6.5 eV. By using this material in the block layer, it is possible to enhance the luminous efficiency. On the other hand, this BCP is poor in the material stability so that it is a large factor to cause a deterioration of the durability.
Also, aluminum(III) bis(2-methyl-8-quinolinato)-4-phenylphenolate (Balq) is frequently used (for example, see Applied Physics Letter, 2002, Vol. 81, page 162). Though this material is excellent in the durability, its T1 is smaller than 2.4 eV, and its ionization potential is small as 5.8 eV. Therefore, Balq is poor in the blocking ability and causes a lowering of the luminous efficiency.
In the light of the above, it is a present state that any block layer having excellent abilities to block an exciton and to block a hole and having excellent durability has not been realized yet.
Recently, there has been reported a method in which a material having a large energy difference Eg between highest occupied molecular orbital and lowest unoccupied molecular orbital is used in the block layer to make the concentration of the block material thin, whereby the luminous efficiency is enhanced (for example, see Organic Electronics, 2003, Vol. 4, page 77). In this method, octaphenylcyclooctatetraene or a hexaphenylene compound is used as the material having a large Eg. However, the Eg of these compounds is 3.3 eV and 3.6 eV, respectively, and therefore, it cannot be said that these compounds have a thoroughly large Eg, and their rate of enhancement of the luminous efficient is small.
Also, there is disclosed a measure for containing an electrically inactive polymer binder (for example, see JP-A-2002-180040). However, since the polymer is used, the production process of an organic electroluminescent device is limited to a wet film formation process, and a dry film formation process such as vapor deposition cannot be applied.